(1) Field of the Invention
The invention relates in general to drill stem threaded connections used in oil field and trenchless horizontal drilling operations. In particular, the present invention relates to threaded connections on drill pipe tool joints, heavy weight drill pipe, drill collars, and or other drill stem elements. Still more particularly, this invention relates to double shouldered connections capable of withstanding increased torque, bending and tensile loading, while maintaining a tool joint with a large ID and a small. OD.
(2) Description of the Related Art
U.S. Pat. No. 5,908,212 (“the Smith patent”) discloses a double-shoulder threaded connection with a taper less than one inch per foot and a box counterbore with length of at least 1.5 inches (FIG. 1, Column 1, lines 34-50). It achieves high torque resistance with a minimum ratio of cross-sectional areas. The Smith patent does not disclose (a) means for resistance to bending fatigue by the connection threads, (b) means for reducing stress concentrations in the connection roots, or (c) a means to avoid swelling or buckling of the box counterbore or pin nose.
U.S. Pat. No. 6,030,004 (“the Schock patent”) discloses a double-shoulder, torque-resistant threaded connection. The tool joint is provided with thread shaving a seventy-five degree included angle between the thread flanks, and with generally elliptical root surfaces (FIG. 1 Column 4, lines 1-23, and Column 5, lines 15-49, FIGS. 7 and 9). The Schock patent does not disclose (a) means for achieving high torque forces with a shallow thread taper; (b) means for enhanced fatigue resistance, using a large root surface that is a product of only a single root radius; and (c) means to achieve a “slim-hole” design, that is, having a small OD and a large ID.
U.S. Pat. No. 7,210,710 (“the Williamson patent”) discloses a double-shoulder drill stem connection. In discussing its FIG. 2, the Williamson patent teaches a thread taper of the box and pin threads of preferably 1 and ⅛ inches per foot. With such a taper, the turns-to-make-up are decreased, because the stabbing depth is increased. However, such a taper decreases the amount of cross-sectional area that is at the secondary (internal) shoulder, which reduces torque capabilities. Also, with such a taper the ID of the connection cannot be as large as shallower taper connections, because there will be a conflict with maintaining enough steel to have an internal shoulder. Finally, such a taper does not allow a “slim-hole” design, that is, a design having a small OD and a large ID.
The Williamson patent teaches the use of dissimilar load flanks. Because of that dissimilarity, the Williamson device has to use two or more radii to bridge the two load flanks. Thus, as claimed in its claim 7, the roots of the internal and external threads are formed in a shape of a portion of an ellipse. The Williamson patent also asserts, in discussing its FIG. 2 (column 8, lines 17-21) that “the length of the pin nose L. sub. PN should be about one to one and one-half times as long as the counterbore length L. sub. BC.” Further, the counterbore length is about 1 inch (longer than conventional connections). The Williamson patent does not disclose (a) means for achieving high torque forces with a shallow thread taper, (b) means for enhanced fatigue resistance using a large root surface that is a product of only a single root radius, and (c) a means to avoid swelling or buckling of the box counterbore or pin nose.
U.S. Pat. No. 5,810,401 discloses dual mating shoulders and nose faces on the pin and box members. U.S. Pat. No. 6,467,818 (“the Snapp patent”), U.S. Pat. No. 6,848,724 (“the Kessler patent”), and U.S. Pat. No. 7,416,374 (“the Breihan patent”) disclose thread forms with one or more root radii larger than that of conventional API by offsetting the center of one or more radii from the centerline of the thread flanks. In each of these designs, offsetting the root radii undercuts one of the thread flanks, weakening the threads' shear capacity, and are limited to radii less than 0.063. All of these patents are incorporated herein by this reference: U.S. Pat. No. 5,810,401; U.S. Pat. No. 5,908,212; U.S. Pat. No. 6,030,004; U.S. Pat. No. 6,467,818; U.S. Pat. No. 6.848.724; U.S. Pat. No. 7,210,710; and U.S. Pat. No. 7,416,374.
Thus, the known prior art for tool joint connections has at least three major deficiencies. The prior art lacks: (1) means for enhanced bending fatigue resistance using a large root surface that does not reduce thread shear capacity, (2) means to achieve a minimal fluid pressure loss and maximum hole cleaning capabilities through a large ID and small OD, while maintaining high torque, bending and tensile load resistance, and (3) means to avoid swelling or buckling of the box counterbore or pin nose.
The performance of a tool joint connection, as a minimum, depends upon the combination of OD, ID, material yield, pitch diameter, truncated thread height, thread angle, threads per inch, taper, one or two shoulders, clearances, and bevel diameters. These parameters are interdependent, so that it is not possible to simply change one at a time without affecting performance.
Prior art of very high torque connections teaches a long box counterbore, and long pin nose lengths are required to provide sufficient deflections to appropriately load both external and internal shoulders. But, a long box counterbore and long pin nose lengths leads to instability (swelling) of these critical sections as described by Smith (paper “Box OD Stability of Double Shouldered Tool Joints at Catastrophic Failure” presented by J. E. Smith et. al. at the March 1996 SPE/IADC Drilling Conference). Applicant has found that in addition to deflections from axial loading of box counterbore and pin nose, significant deflection occurs elsewhere in the connection. Further, applicant has found that the pin nose length should be as short as possible, because the pin nose acts as a bridge between the pin connection and the box internal shoulder for load distribution. That is, the shorter the length of the pin nose, the more compressive stresses the pin nose can take, thus making a stronger connection.